1. Field of the Invention
The present invention relates to a mask for reducing proximity effect, and more particularly, to a mask with a plurality of ladder-shaped assist features for reducing proximity effect. Those ladder-shaped assist features will make the iso space patterns become more line-space patterns like. Not only the process window, but also the image contrast will improve.
2. Description of Related Art
Conventional processes for the fabrication of semiconductor wafers often incorporate a lithographic process to create a desired pattern on a silicon wafer. In the photolithography technology, a limitation R for representing a dimension of a minimum pattern is represented by the Rayleigh formula:R=k1λ/NAWhere the k1 is a constant depending upon the photolithography process;    λ is a wavelength of an exposure light; and    NA is a numeral aperture of an exposure system.    Therefore, the higher NA, the better R.    On the other hand, a depth-of-focus (DoF) is represented byDoF=k2λ/(NA)2Where the k2 is a constant depending upon the photolithography process;
Therefore, the higher NA, the smaller DoF. Due to serious impact to process window, the limit resolution R cannot be reduced by only enlarging NA. In order to reduce the limit resolution without changing NA, various super resolution techniques have been developed. One is to improve illumination systems from conventional mode to annular, quasar or dipole mode. Another is to improve the iso-focal curve by adding assist feature, especially for iso line or iso space.
As the size of the critical dimensions (CD) of the desired pattern shrinks and approaches the resolution value of the lithography equipment, the consistency between the masked and actual layout pattern developed in the photoresist on the silicon wafer is significantly reduced. Specifically, it is observed that differences in pattern development of circuit features depend upon the proximity of the features to one another. Proximity effect in a lithographic process can arise during exposure, resist pattern formation and subsequent pattern transfer steps such as etching. The magnitude of the effect depends on the proximity of the two features present on the masking pattern. Proximity effect is known to result from optical diffraction in the projection system. This diffraction causes adjacent features to interact with one another in such a way to produce pattern-dependent variations.
One type of proximity effect seen in lithographic process is that features having same dimensions on a mask transfer more differently than other features (i.e., have different dimensions) depending on their proximity (i.e. relatively isolated or densely packed) to other features. U.S. Pat. No. 5,242,770 discloses a technique for reducing this disparity by placing non-resolvable scattering bars, also referred to as assist features, next to isolated edges of features such that features having isolated edges and features having densely packed edges transfer approximately the same if their original mask dimensions are the same.
FIG. 1 is a schematic diagram showing a mask 10 disclosed in U.S. Pat. No. 5,242,770. As shown in FIG. 1, the mask 10 includes a plurality of primary features 30–34. The primary feature 32 has isolated edges A1 and A3 and a densely packed edge A2. According to U.S. Pat. No. 5,242,770, assist features 35 and 36 are positioned on two sides of the primary feature 32 and keep a predetermined space from the primary feature 32 so that the isolated edge A3 becomes a corrected edge compared with the isolated edge A1.
According to U.S. Pat. No. 5,242,770, assist features 35 and 36 have a nominal/preferred width, which according to this technique was empirically determined to be ⅕, the critical dimension of the lithographic process and a preferred spacing from edges (also empirically determined) of 1.1 times the critical dimension (for clear field masks). Using, as an example, the 365 nanometer (or 0.365 micron) wavelength of a conventional mercury I-line emission exposure source currently used in the lithographic technology, the nominal scattering bar width is chosen to be 0.1 micron. However, the maximum permissible scattering bar width can be up to or near 0.125 micron such that the scattering bars remain irresolvable.
As the size of the critical dimension shrinks rapidly, the prior art cannot meet the future requirement and possesses the following disadvantages:    1. Without the assist feature design, the process window of the lithographic process cannot meet the product criteria (especially for iso trench or iso line pattern) and the mask error enhancement factor (MEEF) will be too large.    2. With assist feature design such as the technique disclosed in U.S. Pat. No. 5,242,770, the process window may not be enough for product criteria for an even smaller isolation trench design rule. Besides, the location and the number of the assist features are not easy for design.